Many loudspeakers, especially smaller ones, are not capable of faithfully reproducing low-frequency content of an input audio signal. A reason is that the excursion (i.e. displacement) of the membrane is limited. Generally, the sound pressure level L of a loudspeaker depends on the geometry of the loudspeaker and on the frequency f of the electrical excitation signal according to the following relation:
                                          L            ⁡                          (              r              )                                =                      20            ⁢                                                  ⁢                          log              10                        ⁢                          x              m                        ⁢                          S              m                        ⁢                                          ρ                0                                                              p                  0                                ⁢                                  2                                ⁢                r                                      ⁢                          f              2                                      ,                            (        1        )            wherein xm denotes the excursion of the loudspeaker membrane, Sm denotes the area of the loudspeaker membrane, ρ0 denotes the density of air and p0 denotes the reference sound pressure, commonly equal to 20 μPa. From equation 1, it follows that loudspeakers of small size, i.e. small Sm, will have a limited sound pressure level. Especially at low frequencies the sound pressure level can be degraded, having the effect that the reproduction of music with bass can suffer from distortions. Furthermore, overdriven loudspeakers tend to be less power-efficient in that they have a lower ratio of the input power to the output acoustic power.
One approach to avoiding or reducing loudspeaker saturation or distortion, especially at low frequencies, involves frequency attenuation techniques. For example, U.S. Pat. No. 7,233,833 discloses a method which uses a static filter (high-pass or low-shelving) to truncate an audio signal below a predefined frequency. The low-passed signal is fed to a virtual bass unit to generate harmonics of the low-passed signal. The harmonics are added to the truncated signal, and the resulting signal is passed on to the loudspeaker.
Another approach uses an amplitude-adaptive attenuation method in which low frequencies are dynamically attenuated in such a way that the loudspeaker does not saturate. An amplitude-adaptive attenuation is known in the art as compression. Similarly, a compressor is a device for compressing a signal, i.e., for dynamically controlling a gain of the signal (or gains of selected spectral components of the signal). U.S. Pat. No. 5,832,444, for instance, discloses a compressor which is applied to a low frequency band.
Existing solutions for preventing loudspeaker saturation or overdrive effects have some deficiencies. Notably, a static cut-off filter will often attenuate the low frequency spectrum more strongly than necessary. Existing adaptive equalization methods, on the other hand, can result in a perceivable loss of low frequency content.